Fumed silica is a known thickening and thixotropic agent. As little as 1% thereof by weight can make free-flowing liquids viscous or turn a heavy liquid into a paste. By controlling viscosity, fumed silica aids in the manufacture of dripless surface coatings, non-sagging sealants, and adhesives even at elevated temperatures.
It was found that when fumed silica is added to a system containing an amine terminated reactive liquid polymer (ATRLP) and an epoxy resin in sufficient amount to produce the desired thixotropic characteristic, such a system would retard both the gel time and cure. However, when levels of fumed silica were reduced to where the gel time of the ATRLP/epoxy systems was not affected, the systems did not exhibit sufficient thixotropic characteristics. To promote the desired thixotropic property, alternate solid mildly alkaline materials were tried, such as silicate clays and calcium carbonate fillers. Unfortunately, these alternate materials did not impart the sufficient thixotropic property to the ATRLP/epoxy systems. As was expected, however, they did appear to selectively accelerate the cure of the ATRLP/epoxy systems. The next step was to increase levels of fumed silica to impart sufficient thixotropic characteristics at levels which previously retarded the cure of ATRLP/epoxy systems and admix the mildly alkaline materials to accelerate the cure. It was then discovered that thixotropic ATRLP/epoxy systems containing fumed silica and solid mildly alkaline materials did result in compositions having accelerated gel times.